The present invention concerns a free-cutting steel for machine structural use having good machinability in cutting by cemented carbide tools, such as turning with a cemented carbide tool or drilling with a cemented carbide drill. The invention also concerns a method of preparing the free-cunning steel. The steel for machine structural use according to the invention is suitable for material of machine parts produced by machining with cemented carbide tools such as crankshafts and connecting rods, for which abrasion of tools and roughness of turned surface are problems.
In the present invention the term xe2x80x9cdouble structure inclusionxe2x80x9d means inclusions of the structure in which an inclusion consisting mainly of sulfides is surrounding a core of another inclusion consisting mainly of oxides. The terms xe2x80x9ctool life ratioxe2x80x9d and xe2x80x9clife ratioxe2x80x9d mean a ratio of tool life of the free-cutting steel according to the invention to tool life of the conventional sulfur-free-cutting steel containing the same S-contents in turning with a cemented carbide tool.
Research and development on machine structural use having high machinability have been made for many years, and the applicant has made many proposals. In recent years Japanese patent disclosure 10-287953 bearing the title xe2x80x9cSteel for machine structural use having good mechanical properties and drilling machinabilityxe2x80x9d is mentioned as one of the representative technologies. The free-cutting steel of this invention is characterized by calcium-manganese sulfide inclusion containing 1% or more of Ca in a spindle shape with an aspect ratio (length/width) up to 5, which envelopes a core of calcium aluminate containing 8-62% of CaO. Though the steel exhibited excellent machinability, dispersion of the machinability has been sometimes experienced. This was considered to be due to variety of types of the above-mentioned calcium-manganese sulfide inclusion.
The applicant disclosed in Japanese patent disclosure 2000-34534 xe2x80x9cSteel for machine structural use having good machinability in turningxe2x80x9d that, with classification of Ca-containing sulfide inclusions into three groups by Ca-contents observed as the area percentages in microscopic field, A: Ca-content more than 40%, B: Ca-content 0.3-40%, and C; Ca-content less than 0.3%, a steel satisfying the conditions, A/(A+B+C):xe2x89xa63 and B/(A+B+C)xe2x89xa70.1, brings about very prolonged tool life in turning
Further research by the applicant succeeded, as disclosed in Japanese patent disclosure 2000-219936 xe2x80x9cFree-cutting steelxe2x80x9d, in decreasing the dispersion of the machinability by clarifying necessary number of inclusion particles in the steel. The steel of this invention is characterized in that it contains five or more particles per 3.3 mm2 of equivalent diameter 5 xcexcm or more of sulfide inclusion containing 0.1-1% of Ca. There was, however, still some room for improving the dispersion of the machinability.
The object of the invention is not only to clarify the form of the inclusions allowing good machinability, i.e., the above-mentioned double structure inclusions, but also to grasp the effect of manufacturing conditions on the form of the inclusions, and thereby to provide a free-cutting steel for machine structural use which always exhibits desired machinability, particularly, by cutting with cemented carbide tools, as well as the method for producing such a free-cutting steel. In this invention the inventors aimed at such improvement in machinability that achieves fivefold or more in the above-defined tool life ratio.
The free-cutting steel for machine structural use according to the present invention achieving the above-mentioned object, has an alloy composition consisting essentially of, as the basic alloy components, by weight %. C: 0.05-0.8%, Si: 0.01-2.5%, Mn: 0.1-3.5%, S: 0.01-0.2%, Al: 0.001-0.020%, Ca: 0.0005-0.02% and O: 0.0005-0.01%, the balance being Fe and inevitable Impurities, and is characterized in that the area in microscopic field occupied by the sulfide inclusions containing Ca of 1.0% or more neighboring to oxide inclusions containing CaO of 8-62% is 2.0xc3x9710xe2x88x924 mm2 or more per 3.5 mm2.